This invention relates to a data collection apparatus of multi-drop or multi-point connection type including a master station connected to a plurality of slave stations through two unidirectional transmission lines, and more particularly to a data collection apparatus for transmitting the response data from slave stations to the master station with a high speed.
For example, a data collection apparatus including a master station connected to slave stations by multi-drop connection is publicly known in a Japanese Laid-Open Patent Application No. 107487/77 "Data collection apparatus of multi-drop 1 vs. N type for remote monitoring control systems". The master station calls a plurality of slave stations one by one and collects the response data from the slave stations. The minimum time required for calling out the slave station and obtaining the response data T (called period of the slave station) is given by EQU T=t.sub.1 +t.sub.2 +t.sub.3 +t.sub.4
where
t.sub.1 is a slave station call time required for the master station to call for the slave station; PA1 t.sub.2 is a transmission switch-on control time from the switch-on (ON) of the carrier wave of a modulation apparatus to the data transmission by the execution of transmission control of the slave station; PA1 t.sub.3 is a response data transmission time of the slave station; and PA1 t.sub.4 is a tansmission switch-off control time from the end of data transmission of the slave station to the switch-off (OFF) of the carrier wave.
The carrier wave ON/OFF control of the modulator in the slave station is necessary, because the upward transmission line to the master station allows only one carrier wave for the data transmission from a slave station.
That is, when the ON/OFF control of the carrier wave is not executed, the carrier waves for the data transmission from a plurality of slave stations are mixed in the upward transmission line and hence the master station cannot demodulate accurately the response data of the slave station corresponding to a call from the master station. Now, assuming that a modulator-demodulator of 4800 BPS is used, the time required for transmission of 1 bit data is 208 .mu.s. Since the above-mentioned transmission switch-on control time t.sub.2 should be 708 ms according to the advice of CCITT, no high-speed call of each slave station has been effected by the prior art. According to a specific example, when the modulator-demodulator of 4800 BPS is used, the times t.sub.1, t.sub.2, t.sub.3 and t.sub.4 mentioned above are respectively t.sub.1 =12 ms, t.sub.2 =708 ms, t.sub.3 =65 ms and t.sub.4 =2 ms. Therefore, the time T from the call of the slave station to the receipt of the response data from the slave station (hereinafter referred to as a slave station call period) required 787 ms.